The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing NOx emissions is a selective catalyst reduction (“SCR”) device. The SCR device usually includes a substrate, where a SCR catalyst compound is applied to the substrate. A reductant is typically sprayed into hot exhaust gases upstream of the SCR device. The reductant may be a urea solution that decomposes to ammonia (NH3) in the hot exhaust gases, and is subsequently absorbed by the SCR device. The NH3 then reduces the NOx to nitrogen in the presence of the SCR catalyst.
Some types of engines tend to produce cooler exhaust temperatures, especially during engine startup and during moderate operating conditions. For example, highly efficient engines tend to have cooler exhaust temperatures. Cooler exhaust temperatures also tend to occur during low load driving as well. However, cooler exhaust temperatures tend to reduce the effectiveness of the SCR device. This is because a SCR device needs to reach a minimum operating or light-off temperature to convert the urea to ammonia, which is typically about 200° C., to effectively filter NOx. In low temperature environments, an SCR device may not efficiently clean exhaust until several minutes after an engine has been started.
One approach to increasing the effectiveness of the SCR device involves having the engine operate at a higher temperature, which in turn also raises the temperature of the exhaust gases. However, this approach involves the engine operating at a lower level of efficiency to create the hotter exhaust gas, which results in greater fuel consumption. Accordingly, it is desirable to provide an efficient approach to increasing the temperature of the exhaust gases upstream of the SCR device.